Along with technology development and increased demands for mobile devices, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density, high operating potential, a long cycle life and a low self-discharging rate, have been commercialized and widely used.
Secondary batteries are classified into a cylindrical or rectangular battery in which an electrode assembly is included in a cylindrical or rectangular metal can and a pouch-type battery in which an electrode assembly is included in a pouch-type case made of an aluminum laminate sheet, depending on the shape of the battery case.
In addition, the electrode assembly included in the battery case is a power generation device capable of charging and discharging, having a laminate structure composed of a positive electrode, a separator and a negative electrode. The electrode assembly is classified into a jelly-roll electrode assembly in which a positive electrode and a negative electrode with an elongated sheet form are rolled with a separator being interposed between them, a stacked electrode assembly in which a plurality of positive and negative electrodes of a predetermined size are sequentially stacked with a separator being interposed therebetween, and a stacked-folded electrode assembly in which stacked unit cells are wound with a separating film. The electrode assembly of the stacked structure and the electrode assembly of the stacked-folded structure have a common feature in that many electrodes are stacked.
Among them, a general structure of the stacked electrode assembly is shown in FIG. 1.
Referring to FIG. 1, a stacked electrode assembly 10 has a laminate structure composed of a positive electrode, a separator and a negative electrode. Positive electrodes 12, 13 and negative electrodes 22 and 23 have positive electrode tabs 14 and 15 and negative electrode tabs 24 and 25 which are formed to protrude outwards, and the positive electrode tabs 14 and 15 and the negative electrode tabs 24 and 25 are formed in mutually opposite directions.
Also, the plurality of positive electrodes 12, 13 and negative electrodes 22, 23 are stacked with a separator 32 being interposed between the positive electrode 12 and the negative electrode 22. In addition, the positive electrode tabs 14, 15 are coupled to a single positive electrode lead 42, and the negative electrode tabs 24 and 25 are coupled to a single negative electrode lead 44.
In this case, the plurality of positive electrode tabs 14 and 15 and the positive electrode lead 42 are generally coupled to each other by welding. Thus, positive electrode tab welding portions 16 are respectively formed at the positive electrode tabs 14 and 15, and negative electrode tab welding portions 26 are respectively formed at the negative electrode tabs 24, 25.
On the other hand, the positive electrode, the separator and the negative electrode are closely adhered to each other by means of lamination without performing welding separately.
Therefore, in the conventional stacked electrode assembly, the positive electrode, the separator and the negative electrode are closely adhered to each other only through lamination, and thus when a lithium secondary battery capable of repeatedly charging and discharging is used for a long period, the thickness of the entire electrode plates varies due to contraction and expansion of the entire electrode plates during charging and discharging.
Specifically, due to repetitive charging and discharging, a swelling phenomenon may occur among the positive electrode, the separator and the negative electrode, which are laminated.
Accordingly, a non-contact region is formed among the positive electrode, the separator and the negative electrode, which are laminated, and a dendrite is generated in the non-contact region. Also, lithium ions are continuously deposited at the site where the dendrite is generated.
In addition, the thickness of the positive electrode and the negative electrode is gradually increased due to the lithium ions continuously deposited at the site where the dendrite is generated, and accordingly a swelling phenomenon among the positive electrode, the separator and the negative electrode becomes worse. This may increase the thickness of the entire battery cell, and, in some cases, a short circuit occurs due to the dendrite.
Therefore, there is a high need for a technique for preventing a swelling phenomenon among the positive electrode, the separator and the negative electrode, which are laminated, in order to prevent the risk of thickness increase and short circuit of a battery cell while ensuring stable operation of the battery cell.